Move workflows to a dedicated repository for 4.x branch

Author: asenyaev

.github/workflows/PR-4.x-U20.yaml

@@ -0,0 +1,16 @@
+name: PR:4.x
+
+on:
+  pull_request:
+    branches:
+      - 4.x
+
+jobs:
+  ARM64:
+    uses: opencv/ci-gha-workflow/.github/workflows/OCV-Contrib-PR-4.x-ARM64.yaml@main
+
+  U20:
+    uses: opencv/ci-gha-workflow/.github/workflows/OCV-Contrib-PR-4.x-U20.yaml@main
+
+  W10:
+    uses: opencv/ci-gha-workflow/.github/workflows/OCV-Contrib-PR-4.x-W10.yaml@main